Dehydrogenation processes for the conversion of low molecular weight organic compounds to compounds having a higher molecular weight are well known. Continued efforts have been made in recent years to improve such processes in order to improve the conversion rate and selectivity to desired products. The present invention describes a continuous process for the electrocatalytic oxidation of lower molecular weight hydrocarbons such as methane and ethane to higher molecular weight hydrocarbons such as those containing from 2 to 5 carbon atoms. In the present invention, water is a by-product, and electricity is generated.
Electrogenerative and voltameiotic processes are reviewed and compared with conventional electrochemical and heterogeneous processes in Ind. Eng. Chem. Process Dev., Vol. 18, No. 4, pp. 567-579. Oxidation reactions are discussed on page 576.
U.S. Pat. No. 4,329,208 describes the oxidation of ethylene to ethylene oxide in an electrochemical cell which is similar to the electrochemical cell utilized in the present invention.
Otsuka et al in Chemistry Letters, (Japan) pages 319-322, 1985, describe the conversion of methane to ethane/ethylene in an electrocatalytic cell using silver and silver/bismuth oxide as the anode materials. In an earlier publication, Bull. Chem. Soc. Jpn., 57, 3286-3289 (1984), the same authors have described steam reforming of hydrocarbons through a wall of stabilized zirconia which acts as a hydrogen separator. The desired product is hydrogen with a minimum of carbon dioxide, carbon monoxide or hydrocarbons.
More recently, Seimanides and Stoukides reported on the oxidation of methane in a solid electrolyte cell using catalysts such as silver and lithium/magnesium oxide-silver. Ethylene, ethane, carbon monoxide and carbon dioxide were the main products. Electrochemical techniques were applied to increase the selectivity to C.sub.2 products (Preprint, AIChE Meeting, Miami, Fla., November, 1986).
Otsuka et al Chemistry Letters (Japan), 1985, 499-500 describe the selective oxidation of methane to ethane and/or ethylene in a conventional heterogeneous catalytic reactor. A low pressure mixture of oxygen and methane in helium is passed over a metal oxide catalyst at 700.degree. C. Among the metal oxides described as active for this reaction are included rare earth, main group metals and transition metals.
The use of lithium/magnesium oxide catalysts for methane oxidation to ethane/ethylene is described by Ito and Lunsford in Nature, 1985, 314, 721-722. These authors describe passing low pressures of methane and oxygen over lithium carbonate-doped magnesium oxide and producing ethane and ethylene among the products.
Lithium-promoted magnesium oxide catalysts also are described by Ito et al in J. Am. Chem. Soc., Vol. 107, No. 18 (1985), pp. 5062-68.
The electro-oxidation of hydrocarbon fuels commonly derived from coal (e.g., CO, CH.sub.4 and H.sub.2) using a disc of scandia-stabilized zirconia is discussed by R. A. Goffe and D. M. Mason, Journal of Applied Electrochemistry, 11, (1981) 447-452. The anodic face was coated with either porous platinum or gold as the electrode material, and the cathode face was coated with porous platinum. The reactor was operated at 700.degree. C. in one atmosphere in both the self-generated-power (fuel-cell) mode and the applied-power mode. The products of the reacction are described as being CO.sub.2 and water. The authors state that the possibility of side reactions suggest the interesting prospects of obtaining both electrical energy and useful products from the solid electrolyte fuel cells.
The electrocatalytic reactivity of hydrocarbons on a zirconia electrolyte surface is described by B. C. Nguyen, T. A. Lin and D. M. Mason in J. Electrochem Soc.: Electrochemical Science and Technology, September, 1986, pp. 1807-1815.
Numerous publications describe the complete oxidation of methane to carbon dioxide and water in fuel cells. These proceses are not designed to be chemical processes, but rather to generate electricity from a fuel gas and air (or oxygen). The selectivity of these processes is designed for complete combustion rather than partial combustion.